Foil for copper-clad laminates suitable for printed circuit board production have heretofore been made, for the most part, by electrodeposition. The many advantages of this method, including speed of production, economy and a very well developed technology, are, however, offset to a substantial extent by certain disadvantages. A very important disadvantage is the difficulty of producing pinhole-free foils of less than .about.5 microns thickness. Another is the inherent environmental impact of electrodeposition practice. While the pinhole problem may be minimized to some degree by electroplating copper on an aluminum foil carrier surface specially prepared in accordance with the procedure described in U.S. Pat. No. 3,969,199 to Berdan and Luce, it is at the expense of substantially increased processing complexity and cost. In the Berdan et al. process in order to obtain a peel strength that is not in excess of about 2.0 lb/in. the aluminum carrier surface is first given a zincate coating, the zincate coating is substantially all removed by contact with acid and then overplated with a metal, such as zinc.
These shortcomings of the prior art can be avoided through the use of the invention disclosed and claimed in the above-referenced co-pending Ser. No. 499,019.
Then can now also be avoided in still another different way represented by the process of the present invention, which is related to, but different from Ser. No. 499,019 in respects not previously contemplated.
It is a particularly important objective of this invention to be able to utilize certain commercially available aluminum foil as the carrier sheet in the preparation of the laminate precursor leading to the manufacture of copper-clad assemblies for ultimate usage in the preparation of printed circuit boards.
Fully soft aluminum foil is desired in many applications and in making foil for such applications the drawing lubricant can be burned off by heating to 300.degree. C. or even higher. However, in the case of aluminum foil for use as a carrier sheet, retention of the mechanical properties is important. Foil thicknesses employed as carrier sheets are of the order of 0.002 inch thick and must be strong enough to be used in roll form in an unrolling and rerolling operation carried on with a fixed tension. In order to avoid tearing, when such carrier sheet material is separated from the completed copper-clad assembly, it is preferable that the yield strength of the aluminum carrier sheet not be less than about 10,000 psi. This latter, therefore, limits the techniques employable for driving off the surface-contaminating hydrocarbons.
Commercially available chemically cleaned aluminum foil is particularly attractive for this commercially available material retains the full hardness of unannealed severely cold worked aluminum foil. The problem remains, however, that aluminum foil so cleaned still retains on its surfaces a minimum of 0.3 .mu.g/cm.sup.2 of hydrocarbon contaminants, which is enough to rule out van der Waals absorption as a reliable adhesional force between vapor deposited copper and the aluminum carrier sheet on which it is deposited. It is, therefore, necessary to develop other adhesional forces to provide sufficient peel strength in order to have the aluminum foil/copper layer laminate remain intact until it is desired to remove the aluminum by peeling it away. By the practice of this invention it becomes practical to develop such adhesional forces using specially finished commercially available aluminum foil "as received".
Electron beam evaporation is an especially satisfactory method of carrying out the copper deposition step, carrier surface temperature being readily controllable (i.e. by extracting heat) in various ways under such condition and a film of requisite thickness being quickly established uniformly over the carrier surface as required. We also contemplate, however, the possibility of ion plating deposition of the copper film, which would involve biasing the carrier and, if required, introducing an inert gas such as argon into copper vapor to establish the necessary ionization effect. Again, in the case of ion plating, the carrier surface temperature would be amenable to easy control by a variety of alternatives. Induction (RF) evaporation of the copper instead of electron beam evaporation is also contemplated as a means of producing the vapor phase copper required for physical vapor deposition and again, of course, the carrier surface temperature would be readily controllable. If, however, sputtering is the method of deposition to be carried out in the practice of this invention, it will be necessary for special heat removal measures to be taken in order to maintain carrier surface temperature in the temperature range necessary to predetermine the copper/aluminum bond strength within the desired peel strength range.
It will be understood by those skilled in the art that in whatever manner the invention is carried out to provide the copper film on the carrier surface, thereafter one has the choice of proceeding to produce the laminated body.